>> The second process could easily have the page's old TLB entry. It could >> abuse that entry as long as that CPU doesn't context switch >> (switch_mm_irqs_off()) or otherwise flush the TLB entry. > > That is an interesting scenario. Working through this scenario, physmap > TLB entry for a page is flushed on the local processor when the page is > allocated to userspace, in xpfo_alloc_pages(). When the userspace passes > page back into kernel, that page is mapped into kernel space using a va > from kmap pool in xpfo_kmap() which can be different for each new > mapping of the same page. The physical page is unmapped from kernel on > the way back from kernel to userspace by xpfo_kunmap(). So two processes > on different CPUs sharing same physical page might not be seeing the > same virtual address for that page while they are in the kernel, as long > as it is an address from kmap pool. ret2dir attack relies upon being > able to craft a predictable virtual address in the kernel physmap for a > physical page and redirect execution to that address. Does that sound right? All processes share one set of kernel page tables. Or, did your patches change that somehow that I missed? Since they share the page tables, they implicitly share kmap*() mappings. kmap_atomic() is not *used* by more than one CPU, but the mapping is accessible and at least exists for all processors. I'm basically assuming that any entry mapped in a shared page table is exploitable on any CPU regardless of where we logically *want* it to be used.
